This [1] was handy for me, a Pixel user:

[1] https://android.gadgethacks.com/how-to/enable-hidden-graysca...

This is a neat trick. I wonder if my eyes would just get used to see B&W phone content and keep going.

A neat app would be a 5' slow fade off that eventually locks your phone. You can just unlock to keep going but at least you know 5' have gone by.

Thank you. For anyone else on iOS you find this by going to Settings -> Accessibility -> Display & Text Size -> Color Filters -> On. The default color filter when turned on is grayscale.